How is the amount of fat removed in liposuction calculated?

Good question! While techniques vary somewhat, plastic surgeons usually inject approximately the same amount of fluid per area to be treated as they expect to remove as fat. For example if the surgeon estimates that one liter of fat needs to be suctioned from the belly, then one liter of fluid will be injected. After allowing the injected fluid and the medications in it to work for several minutes, then the combination of fat and injected fluid is sucked out, and that is called aspirate, as it is a combination of fat and injected material, and that is what is measured. Not all of the injected fluid is sucked out, as some of it is absorbed by the body. Usually, the estimated amount of fat is sucked out, in this example one liter, plus some fluid. 

During liposuction, tumescent fluid - sterile saline mixed with medications to control bleeding and pain - is injected into the treatment area and allowed to sit for a time.  This tumescent fluid is critical for successful liposuction. It loosens the fat and reduces complications.  Tumescent fluid is then removed along with the fat.  I count the total volume of fluid and tissue removed and never exceed 5 liters in one procedure.  The liposuction aspirate contains fat, tumescent fluid and blood.  Many patients just want to count the amount of pure fat removed, but unfortunately this is impossible to do.  First, there are microscopic amounts of fat mixed in with the fluid.  Also, the risks of liposuction increase when more than 5 liters of aspirate is removed - even though that aspirate contains more than just fat.